High-strength hot-forged aluminum alloy

ABSTRACT

In the microstructure of a hot-forged 6000-series aluminum alloy having a specific chemical composition, grains including small grains with a misorientation of 2° or more are refined, and a KAM, which is an average misorientation of the grains, is controlled within a specific range. This allows the hot-forged 6000-series aluminum alloy to have excellent stress corrosion cracking resistance and still have high tensile strength, high yield strength, and high elongation.

TECHNICAL FIELD

The present invention relates to high-strength, hot-forged aluminum alloys. Hereinafter “aluminum” is also simply referred to as “Al”.

BACKGROUND ART

Reduction in body weight of, and resulting improvements in fuel efficiency of automobiles and other transports have been pursued so as, to cope with global environmental issues caused typically by exhaust gases. To this end, 6000-series (Al—Mg—Si) hot-forged aluminum alloys as prescribed in Aluminum Association (AA) standards or Japanese Industrial Standards (JIS) are used for structural components and structural parts of automobiles and other transports and, in particular, for automobile suspension parts such as upper arms and lower arms. Hot-forged 6000-series aluminum alloys, when used for these structural components and structural parts, offer high strength and high toughness and have relatively excellent corrosion resistances. Hereinafter, such structural components and structural parts of transports will be illustrated by taking automobile suspension parts as an example.

For further weight reduction of automobiles, automobile suspension parts require higher strength and higher toughness, in addition to smaller thicknesses. The automobile suspension parts also function as safety-related parts and require higher corrosion resistance to intergranular corrosion (grain-boundary corrosion) and to stress corrosion cracking so as to ensure reliability as the safety-related parts. Accordingly, various techniques have been developed to improve chemical compositions and microstructures of material hot-forged 6000-series aluminum alloys.

For example, in well known techniques, transition elements having grain refinement effects, such as Mn, Zr, and Cr, are added, or hot forging is performed at a relatively high temperature of about 450° C. to about 570° C., for the grain refinement of forged 6000-series aluminum alloys. In a proposed technique to provide high strength and high toughness, an ingot is once hot-extruded into an extrusion (extruded material), and the extrusion is used and subjected as a material to hot forging into a forged material so as to refine an unrecrystallized region in the microstructure of the forged material (see Patent Literature (PTL) 1).

In contrast, though not in the field of hot-forged materials, metallurgical techniques have been proposed so as to offer higher strength of aluminum alloy materials (see PTL 2 and PTL 3). With these techniques, a 6000-series aluminum alloy ingot is sequentially subjected to solution treatment (solution heat treatment), repeatedly to warm forging at about 150° C. to about 250° C. and to artificial aging.

Also not in the field of aluminum alloys, but in the field of rolled sheets of Corson alloys (Cu—Ni—Si copper alloys), there have been proposed Corson alloys having small anisotropy m strength, having a high yield strength particularly in a direction perpendicular to the sheet rolling direction, and offering bendability in good balance (see (PTL 4 and PTL 5). In these Corson alloys, a kernel average misorientation (KAM) is controlled, where the KAM is an average misorientation of grains and is determined by SEM-EBSD analysis.

The KAM is also publicly known typically in the field of steel sheets as an index for good balance among strength, elongation, and stretch flangeability of high-strength (high-tensile) cold-rolled steel sheets (see PTL 6).

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No.     2011-225988 -   PTL 2: JP-A No. 2014-218685 -   PTL 3: Japanese Patent. No. 5082483 -   PTL 4: Japanese Patent No. 5314663 -   PTL 5: Japanese Patent No. 5476149 -   PTL 6: Japanese Patent No. 4977184

DISCLOSURE OF INVENTION Technical Problem

When an extrusion (extruded material) is used as a material for hot forging as in the technique disclosed in PLT 1, the resulting hot-forged material has a high yield strength in a direction parallel to the extrusion direction, but disadvantageously has high anisotropy in strength.

The grain refinement techniques in the conventional hot-forged 6000-series aluminum alloys are still susceptible to improvements, so as to offer higher tensile strength and higher yield strength.

The techniques of repeatedly performing warm forging on 6000-series aluminum alloy ingots and then performing artificial aging to offer higher strength as proposed in PTL 2 and PTL 3 have been believed to less effectively offer higher strength if hot forging at a higher temperature typically of 500° C. is performed instead of the warm forging. Thus, it is still unknown that these techniques are effective for better mechanical properties of hot-forged 6000-series aluminum alloys.

It is also unknown that the control of the KAM, which is an average misorientation of gains, is effective for better mechanical properties of hot-forged 6000-series aluminum alloys, even if the KAM control is effective for better mechanical properties of rolled sheets of copper alloys or steels as in PLT 3 to 6. This is because the hot-forged 6000-series aluminum alloys are significantly different from these rolled sheets in alloy chemical composition, properties, and production method.

The present invention has been made while focusing on these circumstances and has an object to provide a hot-forged 6000-series aluminum alloy that has excellent corrosion resistance and still has high tensile strength, high yield strength, and high elongation.

Solution to Problem

To achieve the object, the present invention provides a hot-forged aluminum alloy containing, in mass percent, Si in a content of 0.7% to 1.5%, Mg in a content of 0.6% to 1.2%, Fe in a content of 0.01% to 0.5%, and at least one element selected from the group consisting of Mn in a content of 0.05% to 0.8%, Cr in a content of 0.01% to 0.5%, and Zr in a content of 0.01% to 0.2%, with the remainder consisting of Al and unavoidable impurities. In a microstructure in a thickness cent-al part of the hot-forged aluminum alloy, as measured by SEM-EBSD analysis, grains with a misorientation of 2° or more have an average grain size of 30 μm or less, and the grains have a KAM of from 0.6° to 2.0°, where the KAM is an average misorientation of the grains.

Advantageous Effects of Invention

The inventors of the present invention have newly found that not only the grain refinement of a hot-forged 6000-series aluminum alloy, but also the KAM, which results from quantifying the average misorientation of grains, have a strong correlation with the tensile strength and yield strength of this forged material.

The KAM itself represents the quantity of average misorientation of grains measured by SEM-EBSD analysis, and the technique using the KAM is publicly known also as a calculation technique for grain residual strain in other fields than hot-forged 6000-series aluminum alloys, as described in PTL 3 to 6.

The KAM can be advantageously controlled by further subjecting a forged material produced through hot forging to relatively mild forging in a cold to warm region and subsequent artificial aging repeatedly, without changing an already-standardized 6000-sees aluminum alloy chemical composition of the forged material.

The present invention can provide a hot-forged 6000-series aluminum alloy which has high tensile strength, high yield strength, and high elongation without deterioration in corrosion resistance, by refinement of the grains with a misorientation of 2° or more and the control of the KAM. This allows the hot-forged 6000-series aluminum alloy to offer better reliability as a safety-related part in automobile suspension parts.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a side view of a test specimen for stress corrosion cracking resistance evaluation, used in experimental examples; and

FIG. 1B is a plan view of the a test specimen for stress corrosion cracking instance evaluation, used in the experimental examples.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention will be illustrated specifically below.

Chemical Composition

Initially, the chemical composition of an aluminum alloy will be illustrated below, where the aluminum alloy constitutes the hot-forged aluminum alloy (hereinafter also simply referred to as a “hot-forged material” or “forged material”) according to the present invention and constitutes an ingot as a material for the forged material.

The chemical composition of the 6000-series (Al—Mg—Si-series) aluminum alloy for use in the present invention should be determined or specified so as to ensure high strength and high corrosion resistance or durability, typified by stress corrosion cracking resistance, required when the resulting hot-forged aluminum alloy is to be used typically as the forged suspension parts. Accordingly, of chemical compositions within the ranges of 6000-series aluminum alloys, the aluminum alloy for use in the present invention has a chemical composition including, in mass percent, Si in a content of 0.7% to 1.5%, Mg in a content of 0.6% to 1.2%, Fe in a content of am % to 0.5%, and at least one element selected from the group consisting of Mn in a content of 0.05% to 0.8%, Cr in a content of 0.01% to 0.5%, and Zr in a content of 0.01% to 0.2%, with the reminder consisting of Al and unavoidable impurities.

For better properties such as higher strength, the aluminum alloy may thither contain, in mass percent, at least one element selected from the group consisting of Cu in a content of 0.05% to 1.0%, Ti in a content of 0.01% to 0.1%, and Zn in a content of 0.005% to 0.2%. All percentages in contents of elements are mass percent.

Other impurity elements, which are inevitably included into the aluminum alloy typically from scrap as a melting material, are allowed to be contained as the unavoidable impurities of the remainder in the chemical composition, in common amounts on the basis typically of the upper limits prescribed in JIS. Next, critical significance and preferred ranges of contents of the elements will be described.

Si: 0.7% to 1.5%

Silicon (Si) precipitates, with Mg, mainly as a needle-like β′ phase in grains upon artificial aging and is necessary for offering high strength and high yield strength upon use of the hot-forged aluminum alloy as an automobile suspension part.

Si, if present in an excessively low content, may precipitate in an excessively small amount upon artificial aging and may fail to offer high strength.

In contrast, Si, if present in an excessively high content, may cause coarse particles of elementary Si to form and precipitate upon casting and in the course of quenching after solution treatment and may thereby cause the hot-forged aluminum alloy to have lower corrosion resistance and lower toughness. In addition, such a large amount of excessive Si may impede the hot-forged aluminum alloy from having high corrosion resistance, high toughness, and high fatigue properties; and may also adversely affect hot forgeability and workability and may cause deterioration typically in elongation.

For these reasons, the Si content is controlled within the range of 0.7% to 1.5%.

Mg: 0.6% to 1.2%

Magnesium (Mg) also precipitates, with Si, mainly as a needle-like β′ phase in grains upon artificial aging (temper aging) and is necessary for imparting high strength and high yield strength to an automobile suspension part.

Mg, if present in an excessively low content, may precipitate in an excessively small amount upon as trial aging and may fail to offer high strength.

In contrast, Mg, if present in an excessively high content, may cause coarse Mg-containing compounds to be formed m grains and at gain boundaries, and these compounds may adversely affect corrosion resistance and toughness. In addition, such excessive Mg may cause the hot-forged aluminum alloy to have excessively high strength (yield strength), and this may adversely affect not only hot forgeability and workability, but also elongation.

For these reasons, the Mg content is controlled within the range of 0.6% to 1.2%.

Fe: 0.01% to 0.5%

Iron (Fe) forms intermetallic compounds with Si to give dispersed particles (dispersoids). Thus, this element effectively impedes grain boundary migration after recrystallization, restrains recrystallization protects grains from coarsening, and contributes to grain refinement.

In contrast, Fe, if present in an excessively high content, tends to form coarse compounds in grains and at gain boundaries and to cause the hot-forged aluminum alloy to have corrosion resistance and toughness at lower levels. In addition, the intermetallic compounds formed by Fe tend to contain Si, and the formation of these intermetallic compounds may reduce the needle-like β′ phase, because the needle-like β′ phase, which is formed upon artificial aging, requires Si. This tends to cause the hot-forged aluminum alloy to have lower strength.

For these reasons, the Fe content is controlled within the range of 0.01% to 0.5%.

At least one element selected from Mn in a content of 0.05% to 0.8%, Cr in a content of 0.01% to 0.5%, and Zr in a content of 0.01% to 0.2%

As with Fe, manganese (Mn), chromium (Cr), and zirconium (Zr) than, with Si, intermetallic compounds as dispersed particles (dispersoids), impede grain boundary migration after recrystallization, restrain recrystallization, protect grains from coarsening, and effectively contribute to grain refinement.

In contrast, any of Mn, Cr, and Zr, if present in an excessively high content, tend to form coarse compounds in grains and at grain boundaries and to cause the hot-forged aluminum alloy to have corrosion resistance and toughness at lower levels. The intermetallic compounds conned by these elements tend to contain Si, and the formation of the intermetallic compounds reduces the needle-like β′ phase, where the needle-like β′ phase, which is formed upon artificial aging, requires Si. This tends to cause the hot-forged aluminum alloy to have lower strength.

For these reasons, the content(s) of at least one of these elements, when to be contained, is controlled so that the Mn content falls within the range of 0.05% to 0.8%, the Cr content falls within the range of 0.01% to 0.5%, and the Zr content falls within the range of 0.01% to 0.2%.

At least one element selected from Cu in a content of 0.05% to 1.0%, Ti in a content of 0.01% to 0.1%, and Zn in a content of 0.005% to 0.2%

Copper (Cu), titanium (TD, and zinc (Zn) are equieffective elements to allow the forged material to have strength and toughness at higher levels. When these effects are expected, the hot-forged aluminum alloy may contain one or more of these elements selectively.

Cu offers solid-solution strengthening, thereby contributes to higher strength and better toughness of the hot-forged aluminum alloy, and effectively significantly promotes age hardening of the final product upon aging. Cu, if present in an excessively low content, may fail to offer these effects on strength improvements. In contrast, Cu, if present in an excessively high content, may cause the microstructure of the hot-forged aluminum alloy to have significantly high susceptibility (sensitivity) to stress corrosion cracking and to intergranular corrosion and may thereby cause the forged material to deteriorate in corrosion resistance and durability. For these reasons, the content of Cu, when to be contained, may be controlled in the range of 0.05% to 1.0%.

Zn precipitates and forms Zn—Mg precipitates finely in a high density upon artificial aging and allows the hot-forged aluminum alloy to have better strength and toughness. In addition, solute Zn lowers the potential in grains and causes corrosion not to initiate from grain boundaries, but to occur as general corrosion. This effectively results in reduction of intergranular corrosion and stress corrosion cracking. However, Zn, if present in an excessively high content, may cause the hot-forged aluminum alloy to have remarkably lower corrosion resistance. For these reasons, the content of Zn, when to be contained, may be controlled in the range of 0.005% to 0.2%.

Ti effectively refines grains of the ingot, allows the microstructure of the forge material to include fine grains, and allows the forged material to have better strength and toughness. Ti, if pr in an excessively low content, may fail to offer these effects. However, Ti, if present in an excessively high content, may form coarse precipitates to lower the workability. For these reasons, the content of when to be contained, may be controlled in the range of 0.01% to 0.1%.

Elements listed below are impurities and may be contained in contents up to the after-mentioned ranges. Hydrogen tends to be included as an impurity and, particularly when the forged material is worked at a low reduction ratio (working ratio), bubbles derived from hydrogen resist compression bonding in working such as forging and cause blisters, which act as fracture origins. This element thereby causes the hot-forged aluminum alloy to have significantly lower toughness and fatigue properties. In particular, the influence of hydrogen is significant typically in suspension parts designed to have higher strength. Accordingly, the hydrogen content is preferably minimized to 0.25 ml or less per 100 g of Al.

Scandium (Sc), vanadium (V), and hafnium (Hf) also tend to be included as impurities and adversely affect the properties of suspension parts. To eliminate or minimize this, the total content of these elements may be controlled to less than 03%. Boron (B) combines with Ti and allows Ti to more effectively contribute to grain refinement of ingots. Boron, if contained in a content greater than 300 ppm, also forms coarse precipitates and thereby lower the workability. For this reason, the acceptable content of boron is set to be 300 ppm or less.

Microstructure

After controlling the forged material to have an alloy chemical composition within the above-mentioned ranges, the present invention specifies the microstructure of the forged material as follows, where the forged material is for use typically as structural components and structural parts of automobiles and other transports and, in particular, as automobile forged suspension parts. In the microstructure, which is a microstructure of in a thickness central part of the forged material as measured by SEM-EBSD analysis, grains with a misorientation of 2° or more have an average gain size of 30 μm or less and have a KAM of from 0.6° to 2.0° (degree), where the KAM results from the quantification of the average misorientation of the grains with a misorientation of 2° or more.

By the grain refinement and the KAM control as above, the present invention provides the forged material as a hot-forged 6000-series aluminum alloy that has high tensile strength, high yield strength, and high elongation without deterioration in corrosion resistance. The forged material, if having an excessively low KAM of less than 0.6°, may fail to have high tensile strength and/or high yield strength. The forged material, if having an excessively high KAM of greater than 2.0°, may also fail to have high tensile strength and/or high yield strength and, in addition, may have inferior elongation.

As used herein, the term “grains with a misorientation of 2° or more” as measured by SEM-EBSD analysis refers to “gains having boundaries with a misorientation of 2° or more” and includes, in its category, many gains with a misorientation of 2° or more, for example, those with a misorientation of 2°, 15°, or 20°.

It has been found in the present invention that the refinement of grains including even grains with a relatively small misorientation typically of 2° significantly contribute to (affect) higher strength (tensile strength and 0.2% yield strength). On the basis of this finding, the present invention specifies the grains as follows. Specific*, the refinement of the grains with a misorientation of 2° or more to have an average grain size of 30 μm or less allows the hot-forged 6000-series aluminum alloy to have high strength. While the detailed reason thereof has not yet been clarified, this is probably because as follows. Grain boundaries (borders) with a misorientation of 2° or more effectively impede dislocation movement. Thus, the refinement of the grains to have an average grain size of 30 μm or less results in a significantly larger number of the grain boundaries that impede dislocation movement and may allow the forged material to have high strength.

The kernel average misorientation (KAM) in the present invention as measured by SEM-EBSD analysis is the average misorientation of the “gains with a misorientation 2° or more”.

It is publicly known that the KAM itself has a correlation with residual strain, as described typically in Journal of the Society of Materials Science, Japan, Vol. 58, No. 7, pp. 568-574, July 2009.

It is also publicly (mow n that the KAM results from the quantification of local misorientations into an average misorientation, where the local misorientations are each a difference in crystal orientation between adjacent measurement points, as described typically in the patent literature.

The KAM is defined by the formula (Σy)/n, where n is the number of gains; and y is a misorientation (°) of each gain as measured.

The KAM as specified in the present invention differs from conventional equivalents in that objects to be measured for KAM are many pains including even gains with a relatively small misorientation such as gains with a misorientation of 2°, as defined above on gains. Specifically, the RAM significantly varies in value depending on how to specify the misorientations of grains, where the misorientations are the basis of; or the objects of the measurement.

It has been found in the present invention that, not only the refine rent of grains, but also the KAM, resulting from the average misorientation of the “gains with a misorientation of 2° or more” have a strong correlation with the tensile strength and 0.2% yield strength of the hot-forged 6000 series aluminum alloy.

For higher strength, the KAM can be controlled by further subjecting the hot-forged material, which is produced through hot forging repeatedly to a combination process of relatively mild forging in a cold to warm region with subsequent artificial aging, without changing the 6000-series aluminum alloy chemical composition of the hot-forged material, which chemical composition has already been standardized typically for the automobile suspension parts.

Accordingly, a hot-forged 6000-series aluminum alloy having high tensile strength, high yield strength, and high elongation can be produced without deterioration in corrosion resistance and without changes in mechanical properties, where the deterioration and changes are caused by changes in chemical composition and hot forging conditions. This allows the hot-forged 6000-series aluminum alloy to offer better reliability as safety-related parts typically in automobile suspension parts.

In addition, the microstructure and the properties as specified in the present invention can be advantageously achieved even when the hot-forged material is produced through hot forging at a high reduction ratio in terms of minimum reduction in wall thickness of greater than 25%, because the hot forging is not changed in conditions.

For example, a forged suspension part, to which the present invention is applied, generally has a complicated shape as follows. The forged suspension part generally has an approximately triangular shape as a whole and includes an arm portion having an approximately Y shape, and ball joint portions (three portions) disposed at the three ends of the arm portion. The forming of such a complicated shape inevitably requires a high reduction ratio in terms of minimum reduction in wall thickness of greater than 25%. The present invention can provide the specific microstructure and the specific properties even through hot forging performed at such a high reduction ratio.

Site of Measurement by SEM-EBSD Analysis

The average grain size (μm) and KAM of the grains with a misorientation of 2° or more are measured in a thickness central part of the forged material. When the forged material has a simple shape such as a char or cylindrical shape, the thickness central part of the forged material to be measured can be specified on the basis of the center of the forged material. However, the automobile suspension parts representatively have a complicated shape as follows. This shape is an approximately triangular shape as a whole in a plan view. Ball joints at the three apices of the triangle are coupled to each other through arms. The arms each include ribs and a web, where the ribs are in the periphery and have a narrow width and a large thickness, and the web is in the central portion and has a wide width and a small thickness. The arms each have an approximately H- or U-shaped cross section. Accordingly the “Thickness central part” in this case is defined herein as the center of the thickness at any position of the thick ribs, and the grain microstructure in the thickness central part is defined as the measurement object to be measured by SEM-EBSD analysis.

Measurement Method

Specifically, the measurement may be performed in the following manner. Three measurement samples are sampled from any positions of the thickness central part of the thick ribs, and are polished to give MSS sections. A measurement region of 500 μm by 500 μm of the cross section of each sample parallel to the compression direction of the forged material is irradiated with electron beams at a pitch of 1.0 μm using an SEM-EBSD system. The average grain size (μm) of grains with a misorientation of 2° or more, and the KAM resulting from quantification of the average re orientation of the grains are measured, and the three measurements (n=3) are averaged.

The SEM-EBSD (EBSP) analysis is a crystal orientation analysis technique using a field emission scanning electron microscope (FESEM) equipped with an electron back scattering (scattered) diffraction pattern (EBSD, EBSP) analysis s<stem.

More specifically, the samples to be observed by SEM-EBSD analysis may be prepared in the following manner. The observation samples (cross-sectional microstructures) are further mechanically polished and then electrically etched to have a mirror surface. Each of the resulting samples is set in the lens barrel of the FESEM, and electron beams are applied to the mirror surface of the sample to project an EBSP on the screen. An image of this is taken by a highly sensitive camera and captured as an image into a computer. In the computer, the image is analyzed and compared with patterns obtained by simulation on known crystal systems, and on the basis of the comparison, crystal orientations are determined. The determined crystal orientations are recorded as three-dimensional Eulerian angles typically with position coordinates (x, y, z). This process is automatically performed on all measurement points, and gives crystal orientation data at several tens of thousands to several hundreds of thousands of points upon the completion of measurement.

The hot-forged aluminum alloy according to the present invention, which has the alloy chemical composition and the microstructure, preferably has a tensile strength of 420 MPa or more, a 0.2% yield strength of 400 MPa or more, and an elongation of 12% or more. This is preferred in consideration of strength and workability.

Production Method

Next, a method for producing the hot-forged aluminum alloy wording to the present invention will be illustrated. The production process for the hot-forged aluminum alloy in the present invention by itself can be performed by a common procedure, in which an aluminum alloy ingot having the chemical composition is subjected sequentially to homogenization and hot forging to give a forged material, and the forged material is subjected sequentially to solution treatment, quenching, and artificial aging. Namely, the hot-forged aluminum alloy can be produced without a hot extrusion step of the ingot, which is an extra step. However, there are preferred production conditions as follows, so as to allow the resulting hot-forged aluminum alloy to have the microstructure and to have high strength, high toughness, and high corrosion resistance, where the properties are suitable typically for automobile forged suspension pmts.

Casting

Casting of a molten aluminum alloy, which is melted and adjusted to have an aluminum alloy chemical composition within the specific range, may be performed by a common melt casting technique. The melt casting technique may be selected as appropriate typically from continuous casting-directed rolling, semicontinuous casting (direct chill (DC) casting), and hot top casting.

However, the casting of the molten aluminum alloy having an aluminum alloy chemical composition within the specific range is preferably performed at an average cooling rate of 100° C./s or more, for refinement of precipitates and decrease of secondary dendrite arm spacing (secondary DAS).

Homogenization (Soaking)

The homogenization (soaking) of the ingot after casting may be performed by holding the ingot in a temperature range of 450° C. to 580° C. for 2 hours or longer. The homogenization, if performed at a temperature lower than 450° C., may fail to homogenize the ingot due to such excessively low temperature. In contrast, the homogenization, if performed at a temperature higher than 580° C., may cause burring of the ingot surface. Extrusion after homogenization and before hot forging is not necessary, but may be performed when desired.

Hot Forging

The ingot after the homogenization is reheated and subjected to hot forging performed preferably at a material temperature of from 430° C. to 550° C., a forming die temperature of from 100° C. to 250° C., a minimum reduction in wall thickness of 25% or more, and a maximum reduction m v all thickness of 90% or less.

The hot forging may be performed using a mechanical press or using an oil hydraulic press so as to forge the ingot to a final product shape (or a near net shape) of an automobile suspension part. The hot forging may be performed multiple times, including rough forging, intermediate forging, and finish forging, without reheating or with reheating as needed during forging.

The hot forging, if performed at a minimum reduction in wall thickness less than 25% may fail to give the automobile suspension part having the complicated shape with good shape precision by forging, where the minimum reduction in wall thickness is considered as a hot forging reduction ratio. In contrast, the hot forging, if performed at a maximum reduction in wall thickness greater than 90%, may hardly restrain recrystallization and may highly possibly cause coarse recrystallized grains to be formed.

The hot forging, if performed at a forging end temperature after final forging of lower than 300° C., may impede restrainment of recrystallization during forging and solution treatment processes, and this may cause a deformed microstructure to be recrystallized to form coarse grains. These coarse gains, if formed, may impede the forged material from having higher strength and better toughness and may cause the forged material to have lower corrosion resistance, even when the fined material is conformed to have the above-mentioned microstructure. In addition, hot forging, if performed at such a low temperature, may impede refinement of grains in the entire region in a cross section of the forged material. In contrast, the hot forging, if performed at a material temperature higher than 550° C., may highly possibly cause burning of the forged material surface and cause coarse recrystallized gains to be formed.

Solution Treatment and Quenching

The work after the hot forging is subjected to solution treatment and quenching. In the solution treatment, the work is preferably held in a temperature range of 530° C. to 570° C. for a time of 1 hour to 8 hours. The solution treatment, if performed at an excessively low temperature and/or for an excessively short time, may become insufficient and may cause insufficient solid-solution of Mg—Si compounds. This may cause the compounds to precipitate in an excessively small amount in the subsequent artificial aging and cause the forged material to have lower strength. The solution treatment may be performed for a long holding time, but may offer saturated effects when performed for a time longer than 8 hours.

After the solution treatment, the work is preferably subjected to quenching at an average cooling rate of 25° C./s or more in the temperature range of from 500° C. down to 100° C. The cooling in the quenching is preferably performed by water cooling, and particularly preferably performed by water cooling (water tank immersion) in which cooling water is circulated with bubbling. This is preferred for ensuring the above-mentioned average cooling rate and for performing homogeneous cooling to eliminate or minimize strain of the forged material. The quenching treatment, if performed at an excessively low cooling rate, may cause precipitation typically of Mg—Si compounds and Si at grain boundaries and may thereby cause the product after artificial aging to be susceptible to grain boundary fracture and to have toughness and fatigue properties at lower levels. In addition, such quenching treatment at an excessively low cooling rate may cause Mg—Si compounds and Si, which are stable phases, to be formed also in grains in the course of cooling. This may cause a β phase and a β′ phase to precipitate in smaller amounts upon artificial aging and may cause the forged material to have lower strength.

In contrast, the quenching, if performed at an excessively high cooling rate of the forged material is cooled excessively rapidly), may cause hardening strain during quenching to be formed in a large amount, and this may disadvantageously require an extra correcting process after quenching, or may cause the correcting process to include a larger number of steps. In addition, such quenching performed at an excessively high cooling rate gives higher (greater) residual stress and may cause the product to have dimensional precision and shape precision at, lower levels. In consideration of these, the quenching is preferably performed as hot-water quenching at 30° C. to 85° C., at which temperature quenching strain is relaxed. This is preferred for shortening the product production process and lowering the cost. The hot-water quenching, if performed at a temperature lower than 30° C. may cause greater quenching strain. The hot-water quenching, if perforated at a temperature higher than 85° C. may cause the forged material to have toughness, fatigue properties, and strength at lower levels, due to an excessively low cooling rate.

Cold Working or Warm Working

In the present invention, the hot-forged material (after solution treatment and quenching) obtained in the above manner is preferably subjected to a combination process of cold working or warm working with subsequent artificial aging after each working, where the combination process is performed repeatedly at least two times, and where the cold working or warm working is performed at a total reduction in thickness of 5% or more in the temperature range of morn temperature to 200° C. This combination process is performed so as to allow the hot-forged material to have an average grain size and a KAM within the specified ranges.

Assume that the combination process of cold working or warm working with subsequent artificial aging is performed only once, or cold working or warm working is performed even two times but artificial aging is not performed after each cold or warm working. In this case, the resulting hot-forged material may fail to have an average gain size and a KAM both within the specified ranges.

In other words, the repetition of two or mom times of the combination process of cold working or warm working with subsequent artificial aging performed after each working surely allows the resulting forged material to have an average grain size and a KAM of gains with a misorientation of 2° or more both within the specified ranges.

The cold working or warm working, if performed at a low reduction in thickness of less than 5% per one process, may fail to exhibit sufficient effects and may cause the forged material to have a large (coarse) average grain size of grains with a misorientation of 2° or more of greater than 30 μm. The cold working or warm working in this case also tends to cause the forged material to have a low KAM of less than 0.6° and to fail to have the desired high strength.

This is also true in the case where the warm working is performed at an excessively high working temperature of higher than 200° C. Specifically, the warm working in this case may cause the forged material to have a large (coarse) average grain size of grains with a misorientation of 2° or more of greater than 30 μm, to tend to have a low KAM of less than 0.6°, and to fail to have the desired high strength.

In contrast the upper limit of the reduction in thickness per one process of the cold worker or warm working is preferably 50%, and more preferably 40%. The cold or warm working, if performed at an excessively high reduction in thickness to cause excessively large strain, may cause the forged material to have an excessively low elongation due to an excessively high KAM. In addition, the working in this case tends to cause cracking during working.

In this regard, with the techniques disclosed in PLT 2 and PTL 3, significantly large strain is applied to the work in warm working. Specifically, with the technique disclosed in PTL 2, the warm wilting is performed at a wilting ratio in terms of reduction in thickness of greater than 85% by the application of an equivalent strain of 2 or more. With the technique disclosed in PTL 3, an equivalent strain of less than 2 is applied, but, in the working examples, the warm working is performed at a working ratio in terms of reduction in thickness of 55% by the application of an equivalent strain of 0.8. If such significantly large strain is applied, the hot-forged 6000-series aluminum alloy according to the present invention and even the 6000-series aluminum alloy ingots disclosed in PTL 2 and PTL 3 each have a significantly low elongation, although having high strength.

For these reasons, cold working or warm working is performed at a reduction in thickness per one process of preferably 5% to 50%, and more preferably 5% to 40%.

Artificial Aging

After each cold or warm working, artificial aging is performed. The combination process of cold or warm working with subsequent artificial aging is performed repeatedly at least two times. To impede natural aging at mom temperature from proceeding, the artificial aging is preferably performed immediately (typically within cane hour, as a rough reference) after each cold or warm working.

The artificial aging conditions in each process are preferably selected within a temperature range of 40° C. to 250° C. and within a holding time range of 20 minutes to 8 hours.

However, even within the condition ranges, optimum conditions should be selected corresponding to the chemical composition of the forged material and the conditions of previous processes such as hot forging, solution treatment, quenching treatment, and cold or warm working. Assume that the artificial aging is performed under conditions not corresponding to the chemical composition and the previous process conditions and performed at an excessively low or an excessively high temperature, or for an excessively short holding time. In, this case, the artificial aging may impede the forged material from having the desired, specified microstructure and from having high tensile strength, high yield strength, and high elongation.

The homogenization and solution treatment as mentioned above may be performed using an apparatus selected as appropriate typically from air furnaces, induction heating furnaces (induction heaters), and salt-bath furnaces. The artificial aging may be performed using an apparatus selected as appropriate typically from air furnaces, induction heating furnaces, and oil baths.

The forged material according to the present invention, when to be used in or as an automobile suspension part, may be subjected to one or more processes, such as machining and surface treatments, as appropriate before and/or after the artificial aging.

The present invention will be illustrated in further detail with reference to several experimental examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the invention; that various changes and modifications can naturally be made therein without departing from the spirit and scope of the invention as described herein; and that all such changes and modifications should be considered to be within the scope of the invention.

Examples

Next, the present invention will be illustrated with reference to several examples (experimental examples). Forged materials as materials for automobile suspension parts were produced in the following manner. Initially, materials having aluminum alloy chemical compositions given in Table 1 were prepared and subjected to solution treatment and quenching under common conditions. The works (materials) were then subjected sequentially to cold or warm working and subsequent artificial aging under different conditions given in Table 2 and yielded the hot-forged materials. The resulting hot-forged materials were subjected to measurements and evaluations on microstructure, mechanical properties, and corrosion resistance, as presented in Table 2.

Specifically, ingots having chemical compositions corresponding to the hot-forged 6000-series aluminum alloy chemical compositions given in Table 1 were prepared by casting via semicontinuous casting at an average cooling rate of 100° C./s or more, in common in each sample. All the aluminum alloy samples given in Table 1 had, in common, a hydrogen content of 0.10 to 0.15 ml per 100 g of Al.

In common in each sample, the outer surface of each of the aluminum alloy ingots was faced by a thickness of 3 mm and cut into a round rod-like billet having a length of 120 mm and a diameter of 75 mm. The billet was homogenized (soaked) at 520° C. for 5 hours and thereafter cooled via, forced wind cooling using a fan at a moiling rate of 100° C./hr or more.

The ingot after homogenization was subjected to hot forging, in which forging was performed three times down to a final wall thickness via mechanical press forming using upper and lower dies in common in each sample. The forging was performed under common conditions at a forging start temperature in the range of 500° C. to 520° C., a forming die temperature in the range of 170° C. to 200° C., and a wall thickness change of 75% (greater than 25%) in the central part of the forged material.

The hot-forged materials after production had a suspension part shape in common in each sample. The suspension part shape is an approximately triangular shape as a whole in a plan view. Ball joints at the three apices of the triangle are coupled to each other though arms. The arms each include ribs and a web, where the ribs are in the periphery and have a narrow width and a large thickness (height) of 60 mm, and the web is in the central portion and has a wide width and a small thickness (height) of 31 mm. The arms each have an approximately H-shaped cross section.

These forged materials were, in common in each sample, subjected to solution treatment at 550° C. for 5 hours using an air furnace and then subjected to the water cooling (water tank immersion) at an average cooling rate of 25° C./s or more in the temperature mange of from 500° C. down to 100° C.

The hot-forged materials (after solution treatment and quenching) obtained in the above manner were different in average grain size and/or KAM typically by subjecting them to cold working or warm working in combination with subsequent artificial aging two times or only once under different conditions given in Table 2.

The resulting hot-forged materials being different in average grain size and/or KAM of grains as above were subjected to measurements and evaluations on microstructure, mechanical properties, and resistance to intergranular stress corrosion cracking by the following methods. The results of these measurements and evaluations are given in Table 2.

Microstructure

The average grain size and KAM of grains were individually measured by the above-mentioned measurement method. Specifically, samples were sampled from a longitudinal section in any thickness central part of a thick rib in any of the approximately H-shaped arms of the forged material. Of the samples, the average grain sized (μm) and KAM of grains with a misorientation of 2° or more were measured by the procedure mentioned above.

Mechanical Properties

A sample was sampled from the thickness central part at any position of the thick rib of the forged material. From the sample, three tensile test specimens direction) having an outer diameter of 5 mm and a gauge length of 25 mm were prepared at any three points in the longitudinal direction of the sample, on which mechanical properties such as tensile strength (MPa), 0.2% yield strength (MPa), and elongation (%) were measured, and the average of the three measurements (at the three points) was determined.

Stress Corrosion Cracking Resistance

The stress corrosion cracking resistance was evaluated in conformity with the alternate immersion test prescribed in JIS H 8711. FIGS. 1A and 1B area side view and a plan view, respectively, of the test specimen for stress corrosion cracking resistance evaluation (C-ring test specimen for SCC test), including its dimensions. When a stress of 300 MPa was applied, a sample undergoing stress corrosion cracking within a time period shorter than 30 days was evaluated as having poor corrosion resistance (x), and a sample undergoing stress corrosion mucking within a time period from 30 days to shorter than 60 days was evaluated as having good corrosion resistance (◯).

As clearly demonstrated by the data in Tables 1 and 2, Examples 1 and 7 to 12 had chemical compositions within the range specified in the present invention and underwent cold working or warm working and artificial aging under conditions within the preferred ranges. As demonstrated by the data in Table 2, these examples therefore had microstructures as specified in the present invention. Specifically, in the microstructure of the thickness central part, grains with a misorientation of 2° or more had an average gain size of 30 μm or less and a KAM of from 0.6° to 2.0°, as measured by SEM-EBSD analysis.

As a result, the examples have excellent stress corrosion cracking resistance and still have high strength in terms of tensile strength of 417 MPa or more, high yield strength in terms of 0.2% yield strength of 398 MPa or more, and a high elongation of 12.6% or more. Thus, the examples can combine the properties necessary for suspension parts.

In contrast, Comparative Examples 2 to 6 in Table 2 are samples having chemical compositions within the ranges, but being produced via cold working or warm working and artificial aging performed under conditions out of the preferred ranges. These samples (comparative examples) failed to meet any of the conditions specified on microstructure in the thickness central part, where the microstructure is me cured by SEM-EBSD analysis. Specifically, these comparative examples had an excessively large average grain size of grains with a misorientation of 2° or more of gaiter than 30 μm (included coarsened grains), or had an excessively low KAM of less than 0.6°, or had an excessively high KAM of greater than 2.0°.

As a result, Comparative Examples 2 to 6 had, in common, significantly lower tensile strength and 02% yield strength as compared with the examples. Of the comparative examples, the sample having an excessively high RAM of grater than 2.0° also had a lower elongation as compared with the examples.

Comparative Example 2 underwent the combination process of warm working and subsequent artificial aging performed only once. This sample therefore had an excessively large (coarse) average grain size of grains with a misorientation of 2° or more of greater than 30 μm and had an excessively low KAM of less than 0.6°.

Comparative Example 3 underwent the combination process of warm working and subsequent artificial aging performed in this sequence two times, but underwent the warm working performed at an excessively small reduction in thickness (reduction ratio). This sample therefore had an excessively large (coarse) average gain size of gains with a misorientation of 2° or more of greater than 30 μm and had an excessively low KAM of less than 0.6°.

Comparative Example 4 underwent the combination process of warm working and artificial aging performed in this sequence two times, but underwent the warm working performed at an excessively high temperature both in the two processes. This sample therefore had an excessively large (coarse) average grain size of grains with a misorientation of 2° or more of greater than 30 μm and had an excessively low KAM of less than 0.6°.

Comparative Example 5 underwent warm working repeated two times, but underwent no artificial aging after the second warm working. This sample had an average grain size of grains with a misorientation of 2° or more of 30 μm or less, but had an excessively high KAM of greater them 2.0.

Comparative Example 6 underwent the combination process of warm working and subsequent artificial aging performed in this sequence two times, but underwent the artificial aging performed at an excessively high temperature both in the two processes. This sample therefore had an excessively large (coarse) average grain size of grains with a misorientation of 2° or more of greater than 30 μm and had an excessively low KAM of less than 0.6°.

Comparative Examples 13 to 24 in Table 2 employed Alloy Nos. 8 to 18 in Table 1, which are alloys having chemical compositions out of the ranges. These comparative examples were low in one or more of tensile strength, 0.2% yield strength, elongation, and stress corrosion cracking resistance as compared with the examples, even when the comparative examples were produced through cold working or warm working and artificial aging performed under conditions within the preferred ranges, regardless of whether they met the conditions in microstructure in the thickness central part, as measured by SEM-EBSD analysis.

Specifically, samples having high tensile strength and/or high yield strength as with the examples had a lower elongation or significantly lower stress corrosion cracking resistance as compared with the examples. Samples having high elongation or good stress mansion cracking resistance as with the examples had tensile strength and 0.2% yield strength at significantly lower levels as compared with the examples.

Comparative Example 13 had an excessively low Mg content (Alloy No. 8 in Table 1).

Comparative Example 14 had an excessively high Mg content (Alloy No. 9 in Table 1).

Comparative Examples 15 and 16 each had an excessively low Si content (Alloy No. 10 in Table 1). Among them, Comparative Example 16 underwent warm working and subsequent artificial aging performed only once.

Comparative Example 17 had an excessively high Si content (Alloy No. 11 in Table 1).

Comparative Examples 18 and 19 contained none of Mn, Cr, and Zr, or contained one of these elements in an excessively low content (Alloy Nos. 12 and 13 in Table 1).

Comparative Example 20 had an excessively high Mn content (Alloy No. 14 in Table 1).

Comparative Examples 21, 22, 23, and 24 respectively had an excessively high content of Cr, Zr, Cu, and Zn (Alloy Nos. 15, 16, 17, and 18 in Table 1).

These results demonstrate the critical significance of the conditions specified in the present invention on chemical composition and microstructure to give hot-forged 6000-series aluminum alloys that have excellent corrosion resistance and still have high tensile strength, high yield strength, and high elongation.

TABLE 1 Chemical composition of forged 6000-series aluminum alloy Alloy (in mass percent; the remainder: Al) number Mg Si Fe Mn Cr Zr Cu Ti Zn 1 0.76 1.04 0.10 0.31 — — — — — 2 0.85 1.04 0.08 0.15 0.06 0.05 — — — 3 0.69 1.15 0.09 0.65 — — — — — 4 1.00 0.85 0.09 — 0.31 — 0.08 0.06 — 5 0.73 0.89 0.17 — — 0.14 — — 0.02 6 0.69 0.81 0.32 0.26 — — 0.63 — — 7 0.71 1.40 0.08 0.30 0.09 — — 0.02 — 8 0.53 0.97 0.07 0.28 0.05 — — — — 9 1.80 1.15 0.10 0.26 0.09 — — — — 10 1.00 0.65 0.10 0.28 0.06 — — — — 11 0.83 1.84 0.14 0.30 0.09 — — — — 12 0.84 0.95 0.11 — — — — — — 13 0.83 0.95 0.07 0.02 — — — — — 14 0.84 0.96 0.07 1.02 — — — — — 15 0.77 0.96 0.14 — 0.71 — — — — 16 0.79 0.97 0.09 — — 0.47 — — — 17 0.77 0.96 0.14 0.29 0.08 — 1.17 — — 18 0.77 0.99 0.13 0.35 — — — — 0.53

TABLE 2 Production method for forged 6000-series aluminum alloy Cold or warm working and subsequentartificial aging after solution treatment Alloy First working Second working number Working Reduction in Working Reduction in in Table temperature thickness First artificial temperature thickness Second artificial Categoy Number 1 (° C.) (%) aging (° C.) (%) aging Example 1 1 150 10 180° C. for 1 hr 150 10 180° C. for 1 hr Comparative 2 1 180 50 180° C. for 1 hr — — — Example 3 1 180 2 180° C. for 1 hr 180  2 180° C. for 1 hr 4 1 250 10 180° C. for 1 hr 250 10 180° C. for 1 hr 5 1 150 20 180° C. for 1 hr 150 30 — 6 1 150 10 260° C. 0.5 hr 150 10 260° C. for 0.5 hr Example 7 2 100 30 190° C. for 1 hr 180 20 160° C. for 5 hr 8 3 120 20 120° C. for 1 hr 150 30 210° C. for 0.5 hr 9 4 140 20 150° C. for 1 hr 120 30 190° C. for 1 hr 10 5 room 10 150° C. for 1 hr room 20 190° C. for 1 hr temperature temperature 11 6 200 20 180° C. for 1 hr 130  5 60° C. for 5 hr 12 7 60 10 130° C. for 1 hr  60 30 160° C. for 5 hr Comparative 13 8 150 40 150° C. for 1 hr 150  5 190° C. for 1 hr Example 14 9 90 30 100° C. for 5 hr 200 10 160° C. for 5 hr 15 10 120 10 120° C. for 1 hr 150 20 190° C. for 1 hr 16 10 200 50 180° C. for 1 hr — — — 17 11 100 30 100° C. for 5 hr 200 10 190° C. for 1 hr 18 12 130 30 140° C. for 1 hr room 10 180° C. for 1 hr temperature 19 13 120 40 120° C. for 1 hr 150 10 190° C. for 1 hr 20 14 130 5 140° C. for 1 hr  60 30 160° C. for 5 hr 21 15 80 20 100° C. for 5 hr 150 20 190° C. for 1 hr 22 16 150 20 150° C. for 1 hr 180 10 180° C. for 5 hr 23 17 140 10 150° C. for 1 hr 150 20 190° C. for 1 hr 24 18 170 10 180° C. for 1 hr 150 20 190° C. for 1 hr Properties of forged 6000-series aluminum alloy Average grain size of grains with KAM of 0.2% Stress misorientation grains with a Tensile Yield corrosion of 2° or more misorientation strength strength Elongation cracking Categoy Number μm of 2° or more MPa MPa % resistence Example 1 22 0.8 417 398 15.2 ∘ Comparative 2 33 0.5 392 374 16.3 ∘ 3 35 0.4 378 358 15.8 ∘ Example 4 42 0.4 354 331 14.6 ∘ 5 26 2.1 394 388 6.5 ∘ 6 38 0.5 343 316 15.3 ∘ Example 7 18 0.9 458 434 16.3 ∘ 8 16 0.8 450 419 12.6 ∘ 9 18 1.2 422 402 13.1 ∘ 10 16 1.6 431 415 15.7 ∘ 11 15 1.0 463 442 18.8 ∘ 12 18 1.1 451 438 14.6 ∘ Comparative 13 26 0.8 374 355 16.6 ∘ Example 14 14 1.1 420 403 5.9 ∘ 15 18 0.8 368 368 15.2 ∘ 16 32 0.7 378 356 14.7 ∘ 17 16 1.0 427 406 5.7 ∘ 18 40 0.8 377 352 18.6 ∘ 19 36 0.9 391 369 16.8 ∘ 20 14 1.3 388 381 5.1 ∘ 21 16 1.2 380 369 3.7 ∘ 22 18 1.2 381 373 4.5 ∘ 23 18 0.9 453 423 18.3 x 24 22 0.8 406 391 15.3 x

While the present invention has been particularly described with reference to specific embodiments thereof, it is obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.

This application claims priority to Japanese Patent Application No. 2015-123043, filed on Jun. 16, 2015, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention can provide hot-forged 6000-series aluminum alloys that have excellent corrosion resistance and still have high tensile strength, high yield strength, and high elongation. The present invention can therefore enlarge the applications of hot-forged 6000-series aluminum alloys to automobile suspension parts and other parts or components of transports such and has significant industrial value. 

1. A high-strength hot-forged aluminum alloy comprising, in mass percent: Si in a content of 0.7% to 1.5%; Mg in a content of 0.6% to 1.2%; Fe in a content of 0.01% to 0.5%; at least one element selected from the group consisting of: Mn in a content, of 0.05% to 0.8%, Cr in a content of 0.01% to 0.5%, and Zr in a content of 0.01% to 0.2%; and wherein, in a microstructure in a thickness central part of the hot-forged aluminum alloy as measured by SEM-EBSD analysis, grains with a misorientation of 2° or more have an average grain size of 30 μm or less, and grains have a KAM of from 0.6° to 2.0°, where the KAN is an average misorientation of the grains.
 2. The high-strength hot-forged aluminum alloy according to claim 1, further comprising, in mass percent, at least one element selected from the group consisting of: Cu in a content of 0.05% to 1.0%; Ti in a content of 0.01% to 0.1%; and Zn in a content of 0.005% to 0.2%.
 3. The high-strength hot-forged aluminum alloy according to claim 1, wherein the hot-forged aluminum alloy has a tensile strength of 420 MPa or more, a 0.2% yield strength of 400 MPa or more, and an elongation of 12% or more.
 4. The high-strength hot-forged aluminum alloy according to claim 2, wherein the hot-forged aluminum alloy has a tensile strength of 420 MPa or more, a 0.2% yield strength of 400 MPa or more, and an elongation of 12% or more. 